Transport of material between the different subcellular compartments of eukaryote cells often requires carrier vesicles, which bud from a donor organelle and fuse with the recipient one. Rab proteins are low molecular weight guanidine triphosphatases (GTPases) of the Ras superfamily which are localized to the membrane surfaces of organelles. They appear to be involved in the regulation of intracellular vesicular transport in both exocytic and endocytic pathways. They may also be involved in the complex and critical processes of organelle fragmentation and restructuring that occur each cell cycle. Rab proteins cycle between active GTP-bound and inactive GDP-bound conformations.
Newly formed Rab proteins associate with Rab escort proteins (REPs) in the cell cytosol. Rab proteins are then stably isoprenylated by the covalent addition of two 20-carbon geranylgeranyl groups to carboxy-terminal cysteine residues (Khosravi-Far R et al (1991) Proc Natl Acad Sci 88: 6264-6268). Prenylation occurs by Rab geranylgeranyl transferase (GGTase) and is essential for Rab protein function and membrane localization. A deficiency in prenylation of one particular Rab leads to choroideremia, a form of retinal degeneration that may cause blindness (Seabra MC et al (1996) J Biol Chem 270: 24420-24427; Seabra et al (1993) Science 259: 377-381). Each of the more than 30 Rab proteins identified appears to have characteristic intracellular distribution and may function in distinct transport events. REPs help transfer newly prenylated Rab proteins to the appropriate organelle membrane.
The amino acid sequence of Rab proteins reveal conserved GTP-binding domains that are characteristic among Ras superfamily members (Zahraoui A et al (1989) J Biol Chem 264: 12394-123401). GTP binding or conversion from GDP to GTP form occurs en route to the organelle membrane. Experimental evidence shows that GTP-bound Rab proteins are directed into nascent transport vesicles where they interact with SNARE factors, a complex of proteins that direct vesicle targeting and fusion. Following vesicle transport, GTPase activating proteins (GAPs) in the target membrane convert Rab proteins to the GDP-bound form. A cytosolic protein, guanine-nucleotide dissociation inhibitor (GDI) helps return GDP-bound Rab proteins to their membrane of origin.
Rab proteins appear to play a role in mediating the function of a viral gene, Rev, which is essential for replication of HIV-1, the virus responsible for AIDS (Flavell RA et al (1996) Proc Natl Acad Sci 93: 4421-4424). Rab proteins, when overexpressed, can significantly enhance Rev function. Furthermore, mutational analysis suggests that Rev protein has a nuclear signal domain that is necessary for localization into the cell nucleus and is likely to be a Rab protein binding site (Flavell et al, supra).
Both the inhibition of vesicle transport and organelle fragmentation during mitosis are due to an inhibition of vesicle fusion, which occurs while vesicle budding continues. Protein phosphorylation by Cdc2 protein kinase is a key regulatory event in mitosis. Toumikoski T et al has shown that addition of Cdc2 protein kinase to interphase cell extracts inhibits vesicle fusion (1989, Nature 342: 942-945). Furthermore, low GTP-gamma-S concentrations, which are likely to block Rab protein GTPase activity, inhibit the fusion reaction, suggesting that Rab proteins could be mediating this critical cell cycle event. Loss of cell cycle control is a key characteristic of all human cancers.
The discovery of additional Rab and Rab-associated genes and the proteins encoded provides potential agents which are more effective than currently available therapeutic agents in the diagnosis and treatment of choroideremia, AIDS, and cancer. Thus, the new Rab and Rab-associated proteins would satisfy a need in the art by providing new means for the diagnosis, prevention, or treatment of choroideremia, AIDS, and cancer.